In recent years the direct reduction of iron oxide to metallic iron has become a practical commercial reality with increasing worldwide acceptance and production. The direct reduced iron which results from direct reduction of iron oxide has a commercially demonstrated utility in iron and steelmaking and particularly in electric arc furnace steelmaking.
Direct reduced iron, which is sometimes known as sponge iron, is not suited as the principal feed material for steelmaking furnaces other than electric arc furnaces. Other steelmaking processes such as the basic oxygen process and the bottom blown oxygen process require large quantities of hot metal, or molten metal as feed material. Thus, for oxygen furnace feed, it is desired to produce a molten product from a direct reduction furnace.
A known type process gasifies solid fuel in a separate combustion-type gasifier utilizing oxygen and steam for gasification. The gas from the gasifier is then cooled and scrubbed, desulfurized, then utilized in a direct reduction furnace as the source of reductant. An example of this combination of gasifier and direct reduction furnace is described in U.S. Pat. No. 3,844,766. This combination also has a fundamental thermal disadvantage in that approximately 50 percent of the solid fuel is consumed by combustion in the gasifier and only the remaining 50 percent of the fuel value is available as a source of reductant. This combination, although highly efficient in the use of the gas from the gasifier for reduction, requires approximately 4.0 to 5.0 Giga calories of solid fuel per metric ton of solid direct reduced iron product.
An electrically operated vertical shaft furnace is taught by U.S. Pat. No. 1,937,064 in which broken coke, graphite, silicon carbide or other conductors are charged to form a burden. Molten metal is then poured through the burden while electrical current also flows through the burden, thus refining the molten metal. The burden is a stationary granular mass of carbonaceous material which does not flow through the furnace. The burden also is not the material being treated, unlike the present invention.
Langhammer U.S. Pat. No. 3,894,864 purports to teach a shaft furnace for producing molten steel by use of an electric arc. The patent fails to explain the completion of the electric circuit which creates the electric arc. Applicants distinguish from this process by utilizing direct resistance heating of their burden, unlike any known reference, as well as by recirculating spent top gas to act as reductant source.
Other patents which may be of interest to the reader include Elvander et al U.S. Pat. No. 3,948,640 and Gross U.S. Pat. No. 3,948,642.